Brushless motors such as switched reluctance motors are increasingly demanded because they are inexpensive and simple in structure. Those motors incorporate an encoder for outputting a pulse signal in synchronism with the rotation of the rotor, which is rotated by sequentially switching the current supply phase by counting pulses of a pulse signal of the encoder and detecting the rotation position of the rotor on the basis of the encoder count. Since it is possible to detect the rotation position of the rotor on the basis of the encoder count after starting, motors of this type having an encoder are used as drive sources of various position switching devices in which a position switching control (positioning control) for rotating the rotor to a target position by means of a feedback control system is performed (JP-A-2001-271917).
In such a motor incorporating an encoder, a feedback control is performed in such a manner that the rotor is rotated toward a target position by switching the current supply phase on the basis of the encoder count in synchronism with pulses of a pulse signal of the encoder. When the encoder count has reached a target count that is set in accordance with the target position, the feedback control is finished with a determination that the rotor has reached the target position. The rotor is stopped at the target position.
In the above conventional motor feedback control, the current supply phase is switched in synchronism with pulses of a pulse signal of the encoder. Therefore, if the rotation of the rotor is stopped for a certain reason and the encoder comes to output no pulse signal, a problem occurs that the current supply phase can no longer be switched and hence the rotor cannot be rotated to the target position.
On the other hand, even if a disconnection occurs in the winding of one phase among the three-phase windings, it is determined that the disconnection has occurred in one of the two systems of drive coils to which the winding of the one phase belongs and the feedback control is continued by using only the drive coil of the other system. However, this configuration requires two systems of drive coils and hence is high in manufacturing cost. In addition, where a feedback control is performed by using only one of the two systems of drive coils, the drive torque is made a half of that of the normal case and insufficient drive torque tends to cause loss of synchronism. As a result, a normal feedback control may not be performed. To secure, with only one drive coil, such high drive torque as enables a normal feedback control, a problem arises that a large drive coil is necessary and the motor becomes large-sized.